Impedance measurements of zinc and amalgamated zinc
electrodes in alkaline electrolyte
Citation for published version (APA):
Hendrikx, J. L. H. M., Visscher, W., & Barendrecht, E. (1985). Impedance measurements of zinc and
amalgamated zinc electrodes in alkaline electrolyte. Electrochimica Acta, 30(8), 999-1006.
https://doi.org/10.1016/0013-4686(85)80163-8
DOI:
10.1016/0013-4686(85)80163-8
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Published: 01/01/1985
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IMPEDANCE
MEASUREMENTS
OF ZINC AND
AMALGAMATED
ZINC ELECTRODES
IN ALKALINE
ELECTROLYTE
J. HENDRIKX*, W. VISSCHER and E. BARENDRECHT
Laboratory for Electrochemistry, Department of Chemistry, Eindhoven University of Technology, P. 0. Box 513, 5600 MB Eindhoven, The Netherlands
(Received 15 October 1984; in revisedform 15 January 1985)
Abstract-The behaviour of zinc and amalgamated zinc electrodes in alkaline zincate electrolytes was studied with the impedance technique in the frequency range 0.01-65,OOO Hz.
The impedance spectra of the amalgamated zinc electrode can be adequately described with a kinetic and diffusion controlled reaction. The results are not affected by the method of amalgamation. The zinc electrode shows a different impedance spectrum indicating that surface processes in which intermediate species such as Zn(OH), Zn(OH);, Zn(OH); are involved, play a more important role.
The i,-values that can be derived from the impedance data were found to be in good agreement with the values obtained with the galvanostatic pulse technique.
1. INTRODUCTION
The use of the impedance technique for the study of electrode processes has expanded rapidly over the last three decades. With this technique information has been obtained on deposition and dissolution phenom- ena of the zinc electrode[l-71.
However, the interpretations of the results differ, in particular with respect to the kind and number of adsorbed intermediate species. Much work has been carried out on the kinetics of the zinc reaction at a mercury electrode, but the amalgamated zinc electrode has not been investigated. Because of the application of zinc in batteries where mostly amalgamated zinc electrodes are employed, the behaviour of zinc and amalgamated zinc electrodes in alkaline zincate elec- trolyte was studied with the impedance method.
2. EXPERIMENTAL.
2.1. Electrodes and cell
The impedance measurements of the disc electrodes were carried out in the three-compartment cell, as given earlier[8].
The electrolyte was an alkaline solution with concentrations ranging from 1.5-I 0 M KOH and 0.014.1 M ZnO. The measurements were performed at 0 and 2000 rpm. The latter rotation frequency was chosen in order to minimize the convective diffusion of the chemical species towards the electrode.
The counter electrode was made of zinc (99.9% Merck) in order to maintain the zincate concentration as constant as possible. As a reference electrode (RE) a Hg/HgO-electrode was used having the same elec- trolyte as used in the cell. All potentials are given with respect to this electrode. In general, the distance between the working electrode and the tip of the
* Present address: AMP Holland, ‘s-Hertogenbosch.
Luggin capillary-RE system was 2 mm in order to avoid shielding of the electrode. Before each meaaure- ment nitrogen was bubbled through the cell to remove dissolved oxygen. All experiments were performed at room temperature (ZOOC).
The electrodes. Several zinc and amalgamated zinc disc electrodes {area 0.28 cm’) were investigated.
The electrode consisted of a polycrystalline zinc rod (99.9 % Merck), machined to 6 mm diameter and embedded in KelF. This type c&electrode construction was also used for the amalgamated zinc electrodes, which consisted of a proprietary zinc rod containing 1 and 6 wt. % mercury, respectively. These pre- amalgamated zinc alloys are manufactured with a special production technique[9] by Metallurgy Hoboken-Overpelt, Belgium. The electrodes with 1 and 6 wt.% mercury are nominated Zn(Hg, I) and Zn(Hg, II) respectively.
A third amalgamated electrode [Zn(Hg, III)] was prepared as follows: the zinc electrode was polished using Sic paper 6OOand then diamond paste 3 pm. The
electrode was amalgamated by immersing for 60 s into a solution of 0.3 g HgClz in 10 g acetone. The resulting black film on the electrode surface was wiped off, the electrode was rinsed with double distilled water and electrochemically etched in the same way as the zinc electrode. Before each experiment the electrode was amalgamated and pretreated anew.
Electrode pretreatment. The zinc and the amalga- mated zinc electrodes [Zn(Hg, I) and Zn(Hg, II)] were first mechanically polished using Sic paper 600. After this treatment, the electrode was eiectrochemically etched in the electrolyte under investigation by varying the potential three to five times from -800 to
- 16OOmV us Hg/HgO, and back. (Scan rate 50 mV s-l.) During this scan much more zinc dis- solved into the soIution than was deposited on the surface. The result was a shiny electrode in which the separate grains could be clearly discerned.
J. HENDRIKX, W. VISSCHER AND E. BARENDRECHT loo0 (II (2) (3) (41 PROCESSOR GENERATOR
I
I
I _
I
I__ --- __I Gv
i ELECTROCHEMICAL INTERFACE MINI. COMPUTER OUT PUT DEVlCES I - - -Fig. 1. System set up for impedance measurements. 1: HP 9816 desktop computer and output devices. 2: Solarrron 1250 frequency response analyser. 3: Solartron 1186 clcctro-
chemical interface. 4: Three-electrode cell.
2.2. Instrumentafion
The apparatus used for the impedance measurements incorporates a frequency response analyser (Solartron 1250), an electrochemical interface (Solartron 1186) and ‘a microcomputer (HP 9X16), which completely
Measurements were made at frequencies from 0.01 Hz to 65 kHz; the amplitude of the sinusoidal controls the experiment. The complete set-up is given
potential perturbation is 1 mV (rms). in Fig. 1.
Most of the impedance diagrams were determined at the rest potential of the zinc electrode; some exper- iments were carried out with anodic or cathodic polarization. 4. - sim ImtR) 1 2. 3. RESULTS
3.1. Amalgamated zinc electrode2
In Fig. 2 a typical impedance spectrum ofthe Zn(Hg, III)-electrode is shown at its rest potential in 7 M KOH/O.I M ZnO in quiescent solution. The spectra of the Zn(Hg, I)- and Zn(Hg, II)-electrodes are similar. This diagram can be described with the Randles equivalent circuit[lO, 1 l] for a simple charge transfer and diffusion-controlled reaction. From the impedance plot the exchange current density (iO), the capacity of the double layer (C,,), the Warburg coef- ficient (a) and the diffusion coefficient of the zincate ion (D) can be calculated, using the formulas [I l]
where
,4 = surface area of the electrode [cm’]
D = diffusion coefficient of the metal ion [cm’ s- ‘1
C& = concentration of metal ion at the electrode surface [mol cmm3]
c& = concentration of metal ion in the bulk phase [mol cm-“]
10 = exchange current [A]
n = electrons per molecule oxidized or reduced a = cathodic transfer coefficient.
As is clear from Fig. 2 the simulated impedance plot
In Table 1 the results of all experiments are tabu- lated as a function of the electrolyte composition and the kind ofamalgamated surface. The table shows that the kind of amalgamation and the concentration of the electrolyte do not noticeably influence the i,- and C,,- values.
for the Zn(Hg, III) eiectrode, shown by the drawn line, is in good agreement with the measured values.
When the disc electrode is rotated, the low frequency domain of the impedance diagram changes. A typical diagram is shown in Fig. 3. (The three amalgamated electrodes produce similar diagrams.) These diagrams can also be described with the Randles circuit, in which now the Warburg impedance has to be corrected for convective transport. This in particular affects the low
0 2 6 I3
--t Rc(Q)
Fig. 2. Measured impedance plot (e) of Zn(Hg, III)-electrode in 7 M KOH/O.l M ZnO at the rest potential
(E, = - 1.382 V vs Hg/HgO)
at
Orpm. Simulated impedance plot (-). Frequency range: 0.01 Hz to 65 kHz (multiplication factor: 2).Impedance measurements of zinc electrodes
Table 1. io. C,,, u and D as function of the solution composition and the type of amalgamated surface
1001
Electrolyte electrode
i0 (A cm-2)
x 104
10 M KOH/O.l M ZnO ZnU-fg, I) 200-290 28-34 1.161.23 1.e2.1
10 M KOH/O. 1 M ZnO Zn(Hg, II) 2-270 30-33 1.02 2.7
10 M KOH/O.l M ZnO Zn(Hg, III) 200-210 25 1.12 2.3
7 M KOH/O.l M ZnO ZnW& 1) 19&260 23-25
7 M KOH/O.l M ZnO ZnWg, 11) 240-294 30-32
7 M KOH/O.l M ZnO Zn(Hg, III) 16C-250 23-24
3 M KOH/O.l M ZnO Zn(Hg, I) 330-650 53-68
3 M KOH/O.l M ZnO Zn(Hg, II) SO-250 23-32
3 M KOH/O.l M ZnO Zn(Hg, III) 175+50 4-4-66
1.5 M KOH/O.Ol M ZnO Zn(Hg, I) 66-102 31-32
1.5 M KOH/O.Ol M ZnO Zn(Hg, II) 63-115 28-39
1.5 M KOH/O.OI M ZnO Zn(Hg, III) -
1.09 1.08 1 .os 0.97 0.92 0.89 2.4 2.4 2.4 i.3 3.6 8.2 4.2 5.2 10.4 Im
II21
t
*-
1
2 3 4 5 - Re [QlFig. 3. Measured impedance plot (e) of Zn(Hg, III)-electrode in 7 M KOH/O.l M ZnO at the rest potential (E, = - 1.382 V vs Hg/HgO) at 2000 rpm. Simulated impedance plot (-). Frequency range: 0.02 Hz to
65 kHz (multiplication factor: 2). frequency part. It can be shown[ 1 I] that at decreasing
frequency the real part becomes
lim Z:, = 06(2/D)“~, (3)
m-0
where S = thickness of the Nernst diffusion layer. The value of 6, obtained from the real part of the Warburg impedance, can be compared with the value, calculated from the Levich equation
6 = 1 61D’/Sw r -i/zy1/6 (4)
where
D =
diffusion coefficient of zincate [cm’ s- ‘JWY = angular frequency of rotation [s-‘1 Y = kinematic viscosity [cm’s_ ‘1.
In Table 2 the &values are summarized.
The agreement between the b-values is good. The simulated impedance diagram (Fig. 3) also agrees well with the measured data.
Superposition of ac-signals on a k-current, whether anodic or cathodic, results in similar diagrams as
Table 2. The Nernst diffusion layer 6 as calculated with Equations (3) and (4) (D and CT are obtained from Table 1 and Y from [IZ]) Electrolyte lim Zb, 0-O (0) S(cm) x lo4 I) Via (cm7 s-l) (cm2 s-“l x 10’) Equation (4)
10 M KOHJO.1 M ZnO 0.8-0.9 1.1 2.3 x 1O-6 2.79 7.8-8.8 8.1
7 M KOH/O. 1 M ZnO 0.7-0.9 1.1 2.4 x 1O-6 1.84 7.fS9.0 7.7
3 M KOH/O.l M ZnO 0.74.8 0.9 3.3 x 10-e 1.23 10.~11.4 8.0
1002 J. HENDRIKX, W. VISSCHER AND E. BARENDRECHT
observed at the rest potential. A typical diagram, at anodic de-current of 10 mA cmm2, is shown in Fig. 4. However, due to the dissolution or deposition process, the surface of the electrode changes continu- ously, thus affecting the impedance measurements. A striking example is given in Fig. 5: at longer deposition times an inductive loop is observed in the impedance diagram, which was found to be typical for a non-
amalgamated zinc electrode as will be shown in Section 3.2.
Therefore, it is not justifiable to obtain kinetic parameters from such measurements. The galvano- static pulse method[8] is more apt for this purpose.
3.2. Non-amalgamated zinc electrode
The impedance spectrum of the pure zinc electrode in 7 M KOH/O.l M ZnO is given in Fig. 6 at the rest potential in quiescent solution. This spectrum is quite different from that of an amalgamated electrode (Fig.
1.5- Im(s21
t 1 -
1 I.5 2 2.5 3 3.5
- RdR)
Fig. 4. Measured impedance plot of Zn(Hg, II)-electrode in 7 M KOH/O.l M ZnO at 2000 rpm. Numbers indicate frequency; i, = 10 mAcm 2.
-
Rc(R)0 DtRECT AFTER PRETREATMENT
Fig. 5. Measured impedance plot of Zn(Hg, II)-electrode in 5 M KOH/O.l M ZnO at 2000 rpm; i,: 10 mA cm-‘; o: directly after pretreatment; l : after ca. 20 mjn deposition.
12- Im( Q)
t 8.
0 4 8 12
’
16 20 26- Re(R)
Fig. 6. Measured impedance spectrum of the zinc electrode in 7 M KOH/O.I M ZnO at 0 rpm (0); simulated impedance plot (---); numbers indicate frequency.
2). The small semicircle at high frequencies is followed by a pattern consisting of a semicircle and a straight line. This part often masks the semicircle at high frequencies.
In the case of heterogeneous reactions, surface processes (such as adsorption, surface diffusion and lattice formation) can play an important role. Based on the adatom mechanism for metal-ion exchange reac- tions[ 131, Fleischmann et al.[14] have proposed an analogous circuitry in which additional components expressing surface adsorption, surface diffusion and lattice formation are included (Fig. 7).
In this circuit C, represents a pseudo-capacitance, Z, the adatom diffusion impedance and Rlat, the impe- dance due to lattice formation. At equilibrium poten- tial these are defined[ 141 by
Z2F2A
Cdl
Fig. 7. Equivalent circuit in which additional components expressing surface processes are included in the Randles
circuit (see text).
where
C. = pseudo capacitance [F]
cx = adatom concentration [mol. cmw2]
k2 = I,/(zFcXA) [s- ‘1
2, = adatom diffusion impedance [S2]
D. = diffusion coefficient along the surface [cm* s- ‘1
ZL = distance between two parallel steplines on the surface [cm].
When w/k, e 1, the Z,-term can be simplified to
Further, we can define R, as
R, = Z, + Rbtt. (8)
The simulated curve based on this equivalent circuit is
shown in Fig. 6 by the drawn line. The agreement is
reasonable taking into account that the surface of the zinc electrode changes somewhat during the measure- ment. The values of iO, C,,, R,, C,, ci [with Equation (3)] and u can be calculated.
In Table 3 all data are summarized for direrent solutions for a series of experiments.
Comparing Tables 3 and 1, it appears that the i,- values are somewhat higher for the zinc than for the amalgamated electrodes, though the scatter is rather large. The double layer capacity is about twice that of the amalgamated zinc electrode. The impedance dia- gram at a rotating zinc-disc electrode, shown in Fig. 8, is quite similar to that of a non-rotating electrode (Fig. 6). Only the Warburg impedance at low frequencies is not observed and it appears that the resistance in the RC-circuit, which relaxes at low frequencies, is about 2-5 times larger than in the absence of rotation.
Table 3. i,, C,,, R,, C, and ci as function of the solution composition at a zinc electrode
Electrolyte i0 @F cm-*)
cdl
(S2k’) (mF%-‘)4
(mol cm- ‘1 Q x 109 (n s-l’Z) 10 M KOH/O.l M ZnO 120-259 -80 3-28 60-70 4-5 7 M KOH/O.l M ZnO 200-750 70-180 4-20 20-80 2-5 3 M KOH/O.l M ZnO 500-l 500 50-100 411 44-120 2-6 1.5 M KOH/O.OI M ZnO 50-l 50 -75 24-34 0.6-1.0 0.2.07 lo-20-
RdQ)
Fig. 8. Measured impedance spectrum of the zinc electrode
in 3 M KOH/O.l M ZnO at 2ooO rpm; numbers indicate frequency.
1004 J. HENDRIKX, W. VISSCHER AND E. BARENDRWHT
Superposition of ac-signals on a de-current changes at the rest potential. It seems that this reduced semi- the impedance diagrams drastically. Figure 9 shows the circle overlaps the semi-circle due to the relaxation of spectra obtained with an anodic de-current of 3.5 (a)
and 35 (b) mAcm- *. Increase of the k-current leads
the R,,C,,-circuit. Also, an inductive and an additional capacitive loop are observed.
to a strong reduction of the large semi-circle observed With a cathodic dc-current similar diagrams are
Im(R)
t
(4) tL
Im(R) I (b) 13 kFig. 9. lmpcdance spectra of a zinc electrode in 7 M KOH/O.l M ZnO at 2000rpm; numbers indicate frequency. a: i, = 3.5 mA cm-‘. b: i, = 35 mA cmm2. ImI 57 1 (b) I
t
2t
l- 0 1 2 3 4 5 - RetRIFig. 10. Impedance spectra of a zinc electrode in 7 M KOH/O.l M ZnO at 2C0O rpm; numbers indicate
observed (Fig. 10). Here also, the large semi-circles shrink with increasing current and an inductive and an
transfer
(R,)
and of lim,,o Zl, (which is propor- tional to a), are affected. The zincate concentration at additional capacitive loop are observed. the surface (<,), which is dependent on the underlyingdc-current, influences the R, and u-values t;ia
Equations (1) and (2).
4. DISCUSSION The striking difference between the impedance
spectra of zinc and amalgamated zinc electrodes clearly The total impedance of the amalgamated electrodes indicates that at zinc surface processes play a more can be adequately described with a model consisting of important role. It was shown in Fig. 6 that the results a kinetic and a diffusion controlled part, if necessary can adequately be described with the analogous cu- corrected for forced convection. Since no difference cuitry of Fig. 7. From the impedance data at high was found between the impedance spectra for Zn(Hg, frequencies &-values were calculated (cf Table 3). I-II-III)-electrodes, it can be concluded that the kind These values are of the same order of magnitude as the of amalgamation has no effect, ie at all three electrodes &,-values obtained with the galvanostatic pulse tech- sufficient mercury is present to cover the surface nique[8]. Table 6 compares the two series of data. The completely. Probably, as a result of the electrode agreement is fair, but the scatter of the datapoints is pretreatment, in which more zinc dissolves than de- large.
posits, concentration of mercury on the surface takes In Table 3 values are given for the surface place, leading to a complete coverage.
The i,-value obtained with the impedance technique
concentration cz as calculated with Equation (5). The number of zinc atoms in the compact plane (OOOl), can be compared with &,-data measured by the authors
with the galvanostatic pulse techniquefg].
where the interatomic distance is 2.66 A[ 121 is 2.7 x 10m9 mol cm-‘.
In Table 4 these &values are compared, showing a From this it follows that in 0.1 M ZnO solutions the
fair agreement. surface of the zinc electrode is completely covered with
The hydroxyl concentration has no appreciable zinc adatoms, so surface diffusion cannot play an influence on the &,-values, as was the case in the important role in the model according to Fleischmann galvanostatic pulse technique. If the &,-values are et aI.[13]. Therefore, the resistance R, must be attri- obtained in direct sequence with the two different buted to R,
,
the resistance of lattice formation techniques at the same Zn(Hg, IIQ-electrode, the [Equation (8)l. I-I owever, this explanation cannot be agreement is excellent. The u-values (Table 1) change satisfactory either, because from the impedance meas- with decreasing hydroxyl concentration which means urements in the presence of a dccurrent (Fig. 9) it can an increase of the diffusion coefficients of zincate (D). be deduced that this resistance decreases strongly withThe D-values agree well with literature data[l5] t(1.57) x lO-6 cm2 s-l (25°C) in the hydroxyl con-
increasing dc-current or overpotential. This is rather unlikely for a resistance due to lattice formation, centration range of 1-12 M].
The impedance diagrams of the amalgamated elec-
particularly in view of the already very high surface
trodes measured with a sine wave superimposed on a
adatom concentration. For this reason, another pro- &-current do not significantly change with respect to
cess which is dependent on the activation potential must be responsible for the resistance
R,.
the diagram at rest potential, ie the shape of the Using an anodic or cathodic de-current we observed diagrams is similar, only the values of the charge an extra inductive and capacitive loop, cf Figs 9 and 10.
This would suggest, according to Epelboin ef a!.[ 1, 163 that intermediates are involved in the total reaction. In Table 4. Comparison of &,-values at amalgamated zinc eke- their model this is mathematical described by
trades calculated oiu different measuring techniques
Electrolyte 10 M KOH/O.l M ZnO 7 M KOH/O.I M ZnO 3 M KOH/O.l M 2110 1.5 M KOH/O.Ol M ZnO &,(A cm-‘) x IO4 Impedance Galvanostatic technique pulse technique 200-290 80-520 60-300 50-100 50650 50-270 60-120 (9) for n intermediates with surface coverage 8,
. 0,.
Epelboin et a!.[ l] observed, during zinc electrocrys- tallization, an impedance diagram with an inductive Faradaic part, characterized by at least three time constants. These are ascribed to the involvement of Table 6. Comparison of &values at the zinc electrode calcu-
lated via different measuring techniques.
i.
(A cm-‘) x lo4 Table 5. Comparison of i,-values, obtained with two dif-ferent techniques at the same electrode surface [Zn(Hg, III)] Impedance Galvanostatic
io(A cm-‘) x IO* Electrolyte technique technique pulse
Electrolyte
10 M KOH/O. 1 M ZnO
Impedance Galvanostatic 10 M KOH/O.l M ZnO 120-290 160-200
technique pulse technique 7 M KOH/O.l M ZnO Zoo-750 30-250
3 M KOH/O.I M ZnO 500-l 500 I Oc-350
2olS210 120-I 50 1.5 M KOH/O.Ol M ZnO 50-l 50
1006 J. HENDRIKX, W. VISSCHER AND E. BARENDRECHT
three intermediate species in the total reaction [Had, Zn,+, and ZnA,, (A is an anion)]. Cachet, Strdder and Wiart[4] pointed out four Faradaic relaxation pro- cesses in the impedance diagrams during steady-state polarization measurements. From these results they concluded 10 at least four adsorbed intermediate species at the interface. The intermediate species, however, were not identified. Following the same reasoning, our data would point to two intermediate species, these intermediates are likely to be zinc-species that are involved in the reaction mechanism[8], such as ZnOH, Zn(OH);, Zn(OH);.
It has been shown by Cachet et al.[l7], that nucleation and growth lead also to an impedance spectrum with both capacitive and inductive loops. Kinetic measurements at zinc and amalgamated zinc electrodes[8] revealed a difference in reaction mechan- ism. This was explained by a difference in adsorption behaviour: adsorbed species at zinc while at the amalgamated electrode only H,O-molecules are present.
The observed impedance spectra likewise indicate a pronounced change of the interface for zinc with respect to amalgamated zinc.
For the explanation of the impedance behaviour therefore a model based on the adsorption of inter- mediates is preferred.
The relaxation processes, observed in[l, 61, take place partly in the very low frequency region (c 0.1 Hz). The question here is whether the impe- dance data measured at such low frequencies (requir- ing very long measuring times) are not influenced by the change of the surface as a result of thedeposition or dissolution process. This change can be seen very clearly during the deposition of zinc onto an amalga- mated zinc surface (Fig. 5). Therefore, not much value should be attached to very low frequency measure- ments (< 0.1 Hz).
5. CONCLUSIONS
The impedance diagrams of zinc and amalgamated zinc electrodes show a remarkable difference.
The impedance diagrams of amalgamated disc elec- trodes are not influenced by the method of amalgam- ation and can be adequately described with a kinetic- and diffusion-controlled reaction. They indicate that
no adsorption of hydroxyl ions or intermediate species takes place at the amalgamated zinc electrode.
The impedance diagrams at the zinc disc electrode indicate that surface processes in which intermediate species such as Zn(OH), Zn(OH);, Zn(OH); are involved play a more important role. For both the zinc and the amalgamated zinc electrodes the derived kinetic parameters are in good agreement with those obtained from the galvanostatic pulse technique.
Acknowledgements-Support for this work by ZWO, the
Netherlands Organization for the Advaneement of Pure Research, and NIVEE, the Netherlands Institute for Elec- troheating and Electrochemistry is gratefully acknowledged.
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